1. Field of the Invention
The present invention relates to a Raman amplification repeater using the amplification function of a signal light caused by Raman scattering which occurs when a pumping light LD is applied to a transmission path fiber and, more particularly, a Raman amplification repeater which enables control of Raman amplification and an optical transmission system using the same.
2. Description of the Related Art
For meeting an increasing demand for communication created as the Internet has been widely used, transmission capacities of optical transmission systems forming a basic communication network have been recently increased at a surprising speed.
In order to cope with such a rapid increase in transmission capacity, optical wavelength division multiplexing techniques (WDM technique) have been established to enable transmission data capacities to be increased.
In a long-distance transmission system, however, expansion of a repeating interval is a target which is hard to achieve as well as scale-up of a transmission capacity.
Under these circumstances, for ensuring a signal to noise power ratio (optical SN ratio) per one wavelength and mitigating transmission waveform distortion caused by the fiber nonlinear effect, proposed is a transmission method, called Raman amplification, of canceling a loss of a transmission path. In this method, in order to realize long-distance transmission, reduction in the fiber nonlinear effect and noise is required, as is flattening of an output spectrum, which is crucial in WDM transmission because a relationship between a gain and pumping light power largely depends on a kind of transmission path and the like.
Structure and operation of a conventional repeater (Example 1 of Conventional Art) using Raman amplification will be described with reference to FIG. 13.
In FIG. 13, wavelengths of pumping light LDs (Laser Diode) 103a, 103b and 103c are 1462.4 nm, 1475.0 nm and 1503.1 nm, respectively, and a signal wavelength band ranges from 1574 to 1609 nm. After being multiplexed by WDM (Wavelength Division Multiplex) couplers 102a and 102b, pumping lights are multiplexed by a pumping light WDM coupler 101 with a signal light on an optical transmission path in the reverse direction to each other. The Pumping light output from a pumping light output point onto the optical transmission path amplifies, in the optical transmission path, a signal light band which is about 13.2 THz apart from the pumping light.
First, by using an appropriate transmission path fiber, output power of the pumping light LDs 103a, 103b and 103c having different wavelengths is obtained so as to have a flat output spectrum after, for example, 7 dB Raman amplification.
Connection to an actual transmission path fiber and operation of the respective pumping light LDs 103a, 103b and 103c by the pumping light output power obtained before the connection to the transmission path is made to conduct Raman amplification.
FIG. 14 shows an output spectrum obtained when Raman amplification is conducted by the above-described conventional Raman amplification repeater. As shown in FIG. 14, with the conventional Raman amplification repeater, since monitor control of an output signal by a PD or the like is not conducted, a control target value of a pumping light LD can not be determined because of a difference in transmission path fibers, or in intra-office losses or the like. As a result, it is difficult to maintain a gain spectrum within a signal band as flat.
Although another system for flattening a Raman amplification spectrum is proposed, which automatically measures gain efficiency of a transmission path to set pumping light power, with such open loop control, when a transmission loss varies due to environment change or the like, or when a signal spectrum slants in the transmission path, it will be difficult to obtain a flat output spectrum.
Description will be made of the Raman amplification repeater (Example 2 of Conventional Art) disclosed in Japanese Patent Laying-Open (Kokai) No. 2000-98433 (Literature 1), which is conventional art for solving the above-described problems. The Raman amplification repeater recited in Literature 1 is characterized as including a pumping light means for generating a plurality of pumping lights and a pumping light power control means for monitoring an input light, or an output light to control each pumping light power based on the monitoring result.
The pumping light power control means recited in Literature 1 controls power of each wavelength light by branching a monitoring light from an output light into a light of a wavelength obtained by adding about 100 nm to a wavelength of each pumping light and monitoring these wavelength lights. The Raman amplification repeater has its peak of a gain at a frequency about 13 THz lower than a frequency of the pumping light, and has a frequency about 13 THz lower as a wavelength about 100 nm longer.
In addition, the pumping light power control means distributes as many monitoring lights branched from an output light as the pumping lights and then monitors each wavelength light which is obtained by adding about 100 nm to a wavelength of each pumping light.
As described in the foregoing, conventional Raman amplification repeaters have difficulty obtaining a flat output spectrum with respect to a light signal within a signal band due to a difference in a transmission path fiber or in intra-office losses or the like.
On the other hand, because the wavelength characteristic control method of optical transmission power by Raman amplifications, as disclosed in Literature 1, controls power of each wavelength light by branching a monitoring light from an output light into a light of a wavelength obtained by adding about 100 nm to a wavelength of each pumping light and monitoring these wavelength lights, it will monitor a light signal whose wavelength is outside of the signal bands, as shown in FIG. 15.
In a case where an amplifier (e.g. EDFA) other than a Raman amplifier is used together, monitoring a signal outside of a signal band results in increasing a loss of the monitoring signal to prevent amplification control from being conducted with high precision, so that it will be difficult to measure an accurate level of the monitoring signal. As a result, controlling an output spectrum within a signal band to be flat will be impossible.